Reset split selection state on animation end instead of success

When overview button tapped in while in overview,
the animation to undo split gets cancelled
(probably because Launcher gets destroyed before the animation
can fully run successfully)

Fixes: 201011797
Test: Same steps in bug don't cause placeholder view
to be shown

Change-Id: I2ea1eac637d6f2a3185d50d01601d3588ec9f535